How to get the filepath to an imported mesh?
I'm not an expert but the import addon gets the folder of the imported file (eg: "basedir = os.path.dirname(filepath)"), but its use is probably temporary to just read the file and other files (original textures?) in the same path, at import time. see 2.79's import_fbx.py "load" method, line 2273.

This original path, if I got it, could be saved in some imported object custom property, to allow later usage, like saving baked texture where the original file was when imported, but I guess it is not, now, so it is lost after the import operation.

So, unless you request (and get) this to be done by the importer maintainer, you could patch it yourself to read that path and save it into a custom property (unfortunately I can't help you with this, I'm not into this). 

Another way could be to write a (custom) import log to a text file (maybe even inside blender text datablocks) so that you can later access it to know where/what you got when you imported the file...

Although, the original folder could (easily) not be "there" anymore, removed or renamed, or it could be readonly... so you can't take it for granted to be able to write there, unless you are following a well defined after-import workflow...

for sure you'll have to check if that (still) exist, is writable, and decide what to do if either things are false.